In recent years, increased concerns about environmental issues have been requiring further improvement of the fuel economy of automobiles. This fuel economy need also applies to rubber compositions for treads of automobile tires. Further, the rubber compositions for treads are required to have abrasion resistance and other properties.
Known methods for improving the fuel economy include a method of reducing the amount of carbon black and adding silica instead and a method of using a certain solution-polymerized styrene-butadiene rubber. However, such rubber compositions containing silica or a large amount of solution-polymerized styrene-butadiene rubber unfortunately exhibit a slow cure rate in vulcanization. If the amount of vulcanization accelerator is increased to increase the cure rate, the resulting rubber compound turns white due to blooming, thereby causing appearance defects. In particular, such formulations with low carbon black content look whiter and thus result in greatly reduced marketability.
Patent Literature 1, for example, discloses a rubber composition for a tread which includes a styrene-butadiene rubber having a hydroxyl group at a molecular chain, polyethylene glycol and a specific type of carbon black and thereby improves the rolling resistance, abrasion resistance, and the like. However, there still remains room for improvement in providing pneumatic tires which can be formed at a good cure rate, and are capable of preventing appearance defects due to blooming and of improving the fuel economy and abrasion resistance in a balanced manner.